Open-wedge high tibial osteotomy (HTO) is an established technique for the treatment of symptomatic varus malaligned knees. In this study, we prospectively followed up 13 patients (14 knees) who underwent navigation system-supported HTO for varus knee deformity. Preoperatively all patients underwent a physical examination, and anteroposterior and laterolateral lower limb weight-bearing digital radiographs were obtained. The following parameters were examined: (1) Insall-Salvati index; (2) posterior tibial slope angle; (3) mechanical femorotibial angle; (4) inferior limb length. At a mean follow-up of 12.6 months, clinical examination showed satisfactory results in all patients. The mean Modified Cincinnati Rating System Questionnaire rating increased significantly from 46.5 ± 7.2 to 84.6 ± 8, while the mean Knee Society Score increased significantly from 51.4 ± 9.9 to 85.1 ± 7.3. The mean Insall-Salvati index changed from 1.11 preoperatively to 1.10 postoperatively (P=.742). According to the navigation system data, the femorotibial mechanical axis was corrected from a varus of 6.3° ± 1.9° to a valgus of 2.1° ± 1.6°. These data matched postoperative radiographic lower limb weight-bearing data. In the group of patients in whom we evaluated the posterior tibial slope with the navigation system, we detected an increase of 0.3° ± 0.4° compared with the preoperative radiographic examination evaluations; the comparison between the postoperative and the preoperative radiograhic examination results showed an increase of 0.5° ± 0.7° (P=.87). The results of our study seem to show greater accuracy of mechanical axis correction and smaller increase in tibial slope when a navigation system is used in open-wedge HTO.